1. Field of the Invention
Apparatuses consistent with the present invention relate to an illuminating apparatus for a flat panel display apparatus, and more particularly, an illuminating apparatus with improved optical efficiency and color purity characteristics including a polarized LGP (LGP) unit having improved polarization performance and providing vertically output light, and a cholesteric liquid crystal filter, and a display apparatus including the illuminating apparatus.
2. Description of the Related Art
Liquid crystal displays (LCDs), which are widely used as flat panel display apparatuses, are non-emissive devices that have liquid crystal injected between an array substrate, including a thin film transistor, and a color filter substrate in order to obtain imaging effects due to a difference between refractive indexes according to an anisotropy of the liquid crystal. Therefore, LCDs require an additional light source, for example, an illuminating apparatus such as a backlight unit.
However, currently used LCDs use only 5% of the light emitted from the light source to display images. The low optical efficiency is caused by light absorption of a polarization plate and a color filter included in the LCD. The LCD is fabricated by disposing two facing substrates on which electric field generating electrodes are respectively formed, and between which liquid crystal is injected. In addition, liquid crystal molecules move due to the electric field generated by applying voltages to the electrodes, and thus, light transmittance varies according to the status of the liquid crystal molecules. That is, the LCD performs as a shutter that transmits or blocks light by changing a polarization direction of the transmitting light, and thus, the LCD uses linearly polarized light in one direction and includes polarization plates on both surfaces of the LCD. The polarization plate disposed on both surfaces of the LCD is an absorptive polarization plate that transmits light polarized in one direction and absorbs light polarized in another direction. The absorbing of about 50% of incident light by the polarization plate is the largest factor in low light utilization efficiency of the LCD.
In addition, the LCD requires a color filter including the three primary colors of red (R), green (G), and blue (B) in order to display full-color images. In addition, the color filter is formed using dyes or pigments. An absorptive color filter only transmits light corresponding to a transmission band from among the incident RGB light, and absorbs the remaining ⅔ of the light, and thus, causes a decrease in optical utilizing efficiency.
In order to solve the above problem, extensive research is being conducted to increase optical efficiency by substituting another element for the absorptive polarization plate or changing the polarization direction of light incident onto the absorptive polarization plate into the same polarization direction as that of a rear polarization plate. For example, a reflective polarization film having a multi-layered structure such as a dual brightness enhancement film (DBEF) can be attached on an upper surface of a polarization LGP in order to improve the optical utilizing efficiency of the LCD. However, the reflective polarization film is costly, and lacks a polarization changing unit, and thus, limits an increase of the optical utilizing efficiency. Therefore, extensive research is being conducted to develop a polarization LGP that can separate and change the polarization direction of light as required.
In addition, cholesteric liquid crystal color filters using cholesteric liquid crystal that selectively reflects light of corresponding wavelengths, acting as a color filter are being researched in order to substitute for the absorptive color filter.
The cholesteric liquid crystal has a spiral structure, and the reflection wavelength is determined by controlling the spiral pitch. Therefore, the wavelength of light that is to be reflected can be controlled by the distribution of the pitches in a pixel.
The wavelength region of the visible light that can be visible to human beings ranges from 400 nm to 700 nm, and central wavelengths of R, G, and B light are respectively around 650 nm, 550 nm, and 450 nm.
That is, if the cholesteric liquid crystal is controlled to vary between left and right pitches with respect to the central wavelength of each of the R, G, B pixels, left-circularly-polarized light or right-circularly-polarized light may be selectively reflected in the wavelength region corresponding to the difference between the pitches, and thus, a cholesteric liquid crystal color filter can be fabricated.
The cholesteric liquid crystal color filter has high optical utilizing efficiency, and selectively reflects light in the corresponding wavelength. Therefore, a cholesteric liquid crystal color filter can improve the color purity and contrast ratio of a display.
However, in a cholesteric liquid crystal color filter, if the light is obliquely incident, rather than vertically incident, the reflective wavelength band shifts, and thus, undesired wavelength components may be transmitted through the cholesteric liquid crystal color filter. That is, the cholesteric liquid crystal selectively reflects light of a certain wavelength according to the rotating pitch of the cholesteric liquid crystal. Since the thickness of the cholesteric liquid crystal color filter varies according to the incident angle of the light, the pitch of the cholesteric liquid crystal also varies, and the wavelength of the reflected light varies. This problem is manifested as crosstalk when the cholesteric liquid crystal color filter is applied to a display device, and the light incident on the cholesteric liquid crystal color filter must be collimated vertically in order to reduce the above problem.
FIG. 1 illustrates a cross-sectional view of an illuminating apparatus including a related art cholesteric liquid crystal color filter. Referring to FIG. 1, the illuminating apparatus including the conventional cholesteric liquid crystal color filter includes a backlight 12, and a linear polarization plate 11 on a side of the backlight 12 on which a glass substrate 1, an alignment layer 2, a quarter wave plate 3, and a cholesteric liquid crystal layer 4 are sequentially stacked. The linear polarization plate 11 is required in this structure since the backlight 12 does not have a polarization-separation function, and the linear polarization plate 11 is an absorptive type of element that only transmits the linearly polarized light and absorbs the remaining light such that the optical utilizing efficiency decreases. In addition, a unit for collimating the light incident on the cholesteric liquid crystal layer 4 within a small angle is not included.